1. Field of the Invention
The present invention relates to a thin film solar cell, and more particularly, to a thin film solar cell and a method of manufacturing the same that improve energy conversion efficiency.
2. Discussion of the Related Art
In general, solar cells are classified into various types according to a material of a light-absorbing layer. Solar cells may be categorized into silicon solar cells having silicon as a light-absorbing layer, compound thin film solar cells using CIS(CuInSe2) or CdTe, III-V group solar cells, dye-sensitized solar cells, and organic solar cells.
Among the solar cells, silicon solar cells include crystalline solar cells and amorphous thin film solar cells. Bulk-type crystalline solar cells are widely used. However, the crystalline solar cells have increasing production costs due to expensive silicon substances and complicated manufacturing processes.
Recently, by forming a solar cell of a thin film type on a relatively low cost substrate, such as glass, metal or plastic, instead of a silicon wafer, researches for reducing the production costs have been made.
A thin film solar cell according to the related art will be explained hereinafter with reference to accompanying drawings.
FIG. 1 is a cross-sectional view of a thin film solar cell according to the related art. In FIG. 1, the related art thin film solar cell 5 includes first and second substrates 10 and 20 of glass or plastic, which are attached with and face each other. A transparent conductive layer 30 is formed on an inner surface of the first substrate 10. A light-absorbing layer 40, which has a p-i-n structure and includes a p-type silicon layer 40a, an i-type silicon layer 40b and an n-type silicon layer 40c, is formed on the transparent conductive layer 30. A reflection electrode 50 is formed on the light-absorbing layer 40 at each of the unit cells C. A polymeric material layer 60 is formed on an inner surface of the second substrate 20 and contacts the reflection electrode 50 of the first substrate 10.
Here, the reflection electrode 50 is formed of one selected from a conductive material group including a material that has relatively high reflectance such as aluminum (Al) and silver (Ag). The reflection electrode 50 maximizes scattering properties of light passing through the light-absorbing layer 40 by reflecting the light.
Although not shown in the figure, an adhesive layer may be formed between the polymeric material layer 60 and the reflection electrode 50 to firmly attach the first and second substrates 10 and 20.
In the thin film solar cell 5, external light incident on the first substrate 10 from the outside passes through the first substrate 10 and the p-type silicon layer 40a and is absorbed by the i-type silicon layer 40b. Electrons and holes are generated in the i-type silicon layer 40b due to the light having a larger energy than a band gap energy of silicon. The electrons and the holes in the i-type silicon layer 40b are diffused to the p-type silicon layer 40a and the n-type silicon layer 40c, respectively, due to an internal electric field and are provided to an external circuit through the transparent conductive electrode 30 and the reflection electrode 50, respectively. According to this, solar energy can be converted into electrical energy.
However, the related art thin film solar cell 5 has a limitation on increasing energy conversion efficiency because it absorbs light with a short wavelength within a range of 200 nm to 800 nm. To improve the energy conversion efficiency, a design that the transparent conductive layer 30 has an uneven surface has been suggested so that the light absorption is increased.
FIG. 2A is a cross-sectional view of schematically illustrating a thin film solar cell including a transparent conductive layer with an uneven surface according to the related art, and FIG. 2B is a view of showing a path of light in the thin film solar cell of FIG. 2A. Here, in FIGS. 2A and 2B, the same parts as FIG. 1 may have the same references, and explanation for the same parts will be omitted.
As shown in FIGS. 2A and 2B, the transparent conductive layer 30 having an uneven surface is formed on the first substrate 10. That is, the transparent conductive layer 30 has peaks and valleys alternating each other at its top surface. The uneven surface of the transparent conductive layer 30 increases and improves conversion efficiency of solar energy.
However, in the thin film solar cell 5, there is light loss when the external light passes through the first substrate 10 and the transparent conductive layer 30. Accordingly, even though the energy conversion efficiency is increased due to the increase of light-scattering and light-transmitting distances at the surface of the transparent conductive layer 30, the light loss may be caused by reflection at a interface between the transparent conductive layer 30 and the light-absorbing layer 40, and the energy conversion efficiency of the thin film solar cell 5 is insufficiently improved.
That is, as shown in FIG. 2B, when the transparent conductive layer 30 has the uneven surface, the scattering properties at the surface are improved. However, when the light passes through the interface between the transparent conductive layer 30 and the light-absorbing layer 40, there is light loss due to reflection and absorption. In the thin film solar cell 5, about 25% of a total loss of the energy conversion efficiency may result from the light loss due to the reflection and absorption at the interface between the transparent conductive layer 30 and the light-absorbing layer 40.